The present invention relates to orthopaedic implants in general, and more particularly to a prosthesis used in conjunction with a proximal spacer.
In orthopaedic implantation surgery, an enlarged canal is created through the reaming of a bone canal and removal of bone tissue. Bone cement is then inserted into the canal. The implant must be quickly inserted into the canal with the proper alignment, before the bone cement cures. It is desirable to provide a prosthesis that can be easily implanted in the correct position and will form a strong bond within a bone canal with the supplied bone cement.
It is known to provide implants with a coating of material to help bonding with bone cement in a bone canal. For example, in U.S. Pat. No. 4,283,799 to Pratt, Jr. et al., a pre-coated hip prosthesis is disclosed. All or most of the surface of the prosthesis is covered with a material that is bonded to the prosthesis. Similarly, U.S. Pat. No. 4,491,987 to Park discloses a prosthesis with a uniform polymer coating on substantially the entire surface of the prosthesis. In addition, U.S. Pat. Nos. 4,281,420 and 4,336,618 to Raab provide a prosthesis coated with a polymethylmethacrylate (PMMA) film fixedly adhered to the surface. The film is applied to the prosthetic surface after treating, and then the film is annealed. Similarly, U.S. Pat. No. 4,365,359 to Raab utilizes a silane coupling agent to adhere the PMMA film to a prosthetic element and further enhance the adherence of the prosthesis. A coating of bone cement compatible polymer around the proximal portion of a prosthesis is disclosed in U.S. Pat. No. 4,881,536 to Noble et al. However, the solid coating incorporates stresses through the design, creating a tendency of cracking or disattachment.
Another technique to increase the ability of the prosthesis to adhere to the bone cement is the use of a textured surface on the prosthesis. An example of such a surface is shown in U.S. Pat. No. 4,795,472 to Crowninshield et al.
While the pre-coats and textured surfaces described above may help a prosthesis better adhere to the bone cement and bone, they do not provide other useful features. Such features include pressurizing the bone cement as the prosthesis is inserted and aligning the prosthesis centrally within the bone canal.
Separate spacers for centralizing the prosthesis in the bone canal are disclosed in the art, for example in U.S. Pat. No. 4,827,919 to Barbarito et al. In use, such spacers are placed over the proximal opening of the bone canal either immediately before or after the bone cement is injected into the canal. The prosthesis is then inserted through the spacer into the canal. Such spacers have the disadvantage that, as the prosthesis is inserted, the bone cement is forced out of the canal and obscures the surgeon's view of the spacer, such that it is difficult to determine whether the prosthesis is properly seated.